1. Field of the Invention
The present invention relates to a wiring board with lead pins and a lead pin, and more particularly to a wiring board with lead pins and a lead pin used for the wiring board, the wiring board being formed by attaching the lead pins to electrode pads formed on a surface of the board such as a pin grid array (PGA) type wiring board.
2. Description of the Related Art
FIGS. 6A to 6C show a related manufacturing process for manufacturing a wiring board with lead pins by attaching lead pins 5 to electrode pads 12 formed on a surface of a wiring board 10. FIG. 6A shows a state in which solder paste 14a is applied to surfaces of the electrode pads 12 formed on the wiring board 10, and the lead pins 5 are arranged so as to be aligned with the electrode pads 12 respectively. A surface of the wiring board 10 on which the electrode pads 12 are formed is coated with a solder resist 16, and the solder resist 16 has circular shaped opening portions in parts corresponding to the electrode pads 12 to which the lead pins 5 are attached. Protective plating such as gold plating is applied to the opening portions on the electrode pads 12.
FIG. 6B shows a state in which the lead pins 5 are attached to the electrode pads 12 by melting the solder paste 14a with reflow heating. The lead pins 5 are pins (flat head pins) that are generally used, whose head portions 5a are formed in disk shapes.
The solder paste 14a melts and goes up to the head portions 5a of the lead pins 5 in a meniscus shape by the reflow heating so that the lead pins 5 are attached to the electrode pads 12. However, during the reflow heating, flux composition or the like included in the solder paste 14a is volatilized, and the volatilized gas may not be released from the solder paste 14a. Thus, the volatilized gas may remain as voids 14b in the attached portions of the lead pins 5 and the electrode pads 12.
After the lead pins 5 are attached to the wiring board 10, when the wiring board 10 (package) is reheated so as to mount semiconductor elements or circuit components on the wiring board 10, gas or air remaining as the voids 14b in a solder 14 expands in the solder 14, or is liable to incline the lead pins 5 or shift the lead pins 5 from their normal positions so as to escape from the melted solder 14. FIG. 6C explanatorily shows a state in which the lead pins 5 are inclined due to the influence of the voids 14b when the wiring board 10 is reheated approximately to a temperature at which the solder 14 melts. Problems are described in JP-A-2001-291815, such that the lead pins 5 are inclined or shifted from the normal positions due to the voids 14b in the solder 14 when the wiring board 10 is reheated after the lead pins 5 are attached to the wiring board 10.
As a method for suppressing the influence of the voids 14b remaining in the solder 14 when the lead pins 5 are attached to the electrode pads 12 by using the solder, an application amount of the solder paste to be applied on the electrode pads 12 is simply reduced. However, when the application amount of the solder paste is reduced, a problem arises that a bonding strength of the lead pins 5 and the electrode pads 12 is deteriorated. Further, in JP-A-2001-291815, as the method for suppressing the influence of the voids 14b, a configuration is described in which grooves are formed on an upper surface of the head portion of the lead pin so that the gas due to the flux composition included in the solder paste or the air included in the solder paste escapes externally along the grooves.
However, in the case where the head portions of the lead pins are formed in flat planes, although the grooves are provided in the head portions, an operation for releasing the volatilized gas due to the flux composition or releasing the air from the solder when reheating the wiring board is not necessarily effective. Further, since a semiconductor element is becoming to have multiple pins, the lead pin is to have an extremely small diameter, for instance, an outside diameter of the head portion of the lead pin is 1 mm or smaller. Accordingly, when the head portions or the like are worked finely on in order to manufacture the lead pins having the extremely small diameter as described above, mass production of the lead pins is difficult to be achieved.
In JP-A-2001-217342, a recessed part is formed on a side surface, which is perpendicular to an end face of a head portion facing a pin attaching portion provided on a board, of the head portion of the lead pin so as to raise the joint strength of the lead pin.